Electronic Control Unit And Engine Management System Engineering Essay
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Published: Mon, 5 Dec 2016
Ever since we have established RHR Engineering workshop, we have carried out several ECU (Electronic Control Unit) mapping. Keeping this in mind, I have decided to make a report on ECU, and its operation for our technicians. This report will instruct our technicians on ECU operation, function, features, and its principles.
The ECU (Electronic Control Unit) or otherwise known as Engine Management System is a computer which controls every action and reaction of the Engine. The ECU consists of an 8-32bit microprocessor, Random Access Memory (RAM), Read Only Memory (ROM), and an Input/Output interface, Analogue & Digital converters and in some cases Electronic Erasable Programmable Read Only Memory (EEPROM). Random Access Memory (RAM) is a flash memory, which is erased once the car is switched off. It provides the ECU with the data which is needed at the moment of operation. Whereas, Read Only Memory (ROM) is permanent memory, therefore it is non-erasable. It stores all the functions, and module of the operating systems e.g. Ignition Timing, Injection Timing. EEPROM is a sophisticated type of memory, which could be found in Aftermarket, and High Valued vehicles. The Function remains the same as ROM, but the function module could be edited or erased by the driver according to his/her desire, via sending commands through on dash buttons or by connecting a Computer to the ECU (only in case of Aftermarket ECU). EEPROM provides the driver with choice of driving style, performance output, and handling characteristics. Analogue & digital converters are used to converts digital signals into analogue signals and vice versa. For example, speed sensors convert digital signal into analogue signal i.e. Speed gauge.
The ECU controls almost all the running gear of the vehicle. The main operating system, which ECU has to control in order to run the engine, is the Fuel Injection System, Ignition Timing, and the Idle Speed Control system. Based on information from the input sensors (engine coolant temperature sensor, Manifold Absolute Pressure sensor, Air Flow sensor etc.), the ECU determines optimum settings for the output actuators (Injectors, Idle Speed Control Valve etc.).
Modern day Vehicles are equipped with sophisticated ECUs. They have several other features such as, Traction Control, Cruise Control, Active Suspension, Electronic Ignition System, Variable Valve Timing activator, and ABS (Anti-Lock Brake System). They also work as diagnostic software, as it houses several Sensors and Actuators around the vehicle.
The ECU operates on information sent by the sensors. If any correction is to be made, or if any function needs to be carried out, it does it using actuators. Input signal are received by the sensors and output signals are given to the actuators. All the information is processed in digital format.
The main function of the ECU is to carry out checks to ensure correct running of the engine. The most common checks are as follows:
These checks are necessary so that the vehicle will meet the emission standards, maintain fuel economy, and be reliable.
The input signals received by the ECU are sent by the Sensors. The sensors act as a stimulus of the ECU. The 3 most commonly found Sensors are described as follows:
1) Air mass meter:
There are 2 types of air mass meter, hot wire type, and vane type. They both serve the same function, although the material used is different. An air mass meter is located after the air filter and before the intake plenum. The air mass meter provides the ECU with an analogue signal that indicates the amount of air being drawn into the engine. The ECU then takes this value to add the correct amount of fuel to maintain the 14.7:1, air: fuel ratio.
The most common type of air mass meter is the HOT Wire type air mass meter. It is mainly used in multi-point injector system. It uses an electrified wire, which acts as a resistor. During operation the wire is heated, but when the air passes over the heated wire, it cools, thus changing the resistance. The ECU monitors the resistance value and can work out how much air is entering the engine.
2) Oxygen/ Lambda Sensor:
The main function of the oxygen sensor is to measure the amount of oxygen content exiting the exhaust. It also serves the purpose to ensure that the air/fuel mixture is chemically correct. An oxygen sensor is fitted between the exhaust manifold and the catalyst converter.
Oxygen sensor has to heat up to approx 350 degrees Celsius to work properly. They send a signal to the ECU via voltage to enable the ECU to maintain, or to correct the air/ fuel ratio. The ceramic tip on the end of the oxygen sensor that protrudes into the exhaust manifold becomes porous and allows the oxygen ions to flow through the tip. This creates the signal voltage, which changes with change in oxygen level inside the exhaust. This signal is therefore transmitted to the fuel injection or to the ECU, which alters the fuel quantity as necessary, until the oxygen content of the exhaust gas is correct for efficient catalytic converter operation, and to maintain fuel economy.
3) Throttle position sensor:
It detects the position of the throttle plate. The ECU maintains the engine idle speed at a given value e.g. 800 revs per minute. The ECU identifies when the engine is at idle from the information supplied by the throttle position sensor.
The throttle position sensor incorporates a potentiometer. The wiper of the potentiometer is connected to the throttle plate, so that as the throttle is moved from the closed, to wide-open, the voltage signal from the sensor changes. The voltage signal is passed to the ECU, and used to determine the exact position of the throttle, and the rate at which the throttle is being opened and closed. It helps the ECU to access driverââ‚¬â„¢s intensions, such as the intention to rapidly accelerate, by detecting the speed or the rate at which the throttle is being opened.
The output signal from the ECU is received by the Actuators. An actuator is a mechanical device which transforms electrical signal into kinetic / mechanical energy. E.g. Injectors, Fuel Pump, idle speed control valve.
There are 3 types of actuators, Solenoid type e.g. Injector, EGR (Exhaust Gas Recirculation Valve) Valve, Stepper Motor type e.g. Idle Speed Control Valve, and a DC (Direct Current) Motor type e.g. Fuel Pump.
The Solenoid type actuator uses a coil of wire around a soft metal, through which when the current passes it produces a magnetic field. This magnetic field is used to convert electrical energy into mechanical energy (linear motion) e.g. Injector
The function of the injector is to deliver a finely atomized spray of fuel to the inlet manifold or the cylinder head. It is located on the inlet manifold after the throttle body.
injector solenoid off
The injector solenoid is connected directly to the battery via a relay; the earth circuit for the injector is connected to the ECU. When the ECU completes the earth circuit the injector circuit is completed. Once the injector circuit is completed, the current is passed to the solenoid (electromagnetic Coil); this in turn produces magnetic energy, and attracts the plunger (needle valve), thus opening the diffuser nozzle, and delivering fuel to the inlet manifold or the combustion chamber.
injector solenoid on
A DC (Direct Current) Motor coverts electrical energy into mechanical energy (rotary motion). The most common example of DC motor type actuator is Fuel Pump.
The DC motor works by electromagnetism. The magnetic force acts perpendicular to both wire and magnetic field. When electric current passes through a coil in a magnetic field, the magnetic force produces torque which turns the DC motor.
The fuel pump supplies the injector with adequate fuel at a sufficient pressure to allow the injectors to give good atomization. In modern day vehicles it is located inside the fuel tank, in order to keep the pump cool, and reduce the noise of operation.
Unlike the Stepper Motor, the fuel pump runs on a DC (Direct Current) Motor, therefore it has a continuous operation, and earth connection. Modern day vehicle are fitted with High Pressure Roller- Cell type fuel pumps. The Roller cell Pump is an example of DC motor, as it converts electrical energy into mechanical energy (rotary motion). When the fuel enters the pump it is compressed by rotating cells which force it through the pump at a high pressure.
The pump can produce a pressure of 8 bar (120 psi. Within the pump is a pressure limiting valve, which maintain the pressure through the fuel pump. The other end of the pump (output) is home to a non-return valve which, when the voltage to the pump is removed, it closes the return to the tank, and maintains pressure within the system. The normal operating pressure within this system is approximately 2 bar (30 psi), at which the current draw on the pump is 3 to 5 amps.
Stepper Motor is an analogue actuator. They are used to convert electrical energy into mechanical energy (rotational movement). Unlike a conventional motor, a stepper motor will rotate but in stages and not full rotations, therefore it can open and close airways partially.
The most common type of stepper motor actuator is an Idle Speed Control Valve.
Idle Speed Control Valve (ISCV):
The Idle Speed Control Valve is used to either regulate the bypass of air around the throttle butterfly or as a throttle motor controlling the position of the throttle butterfly. The ECU constantly alters the control signal to the motor or valve to maintain a constant idle speed.
In an idle speed control valve, a stepper motor is built into the ISCV where it rotates a valve shaft either in or out. This in turn increases or decreases the clearance between the valve and the valve seat, therefore regulating the amount of air allowed to pass through thus controlling the idle. The idle increases when the Air Condition is switched on, this is because of the load on engine received by Air Condition.
Performance of the sensors depends on their location. A hot wire air mass meter will not perform well if it was placed above the exhaust manifold, as it will heat the wire, and give false reading of air drawn in. This error will result in incorrect air/fuel mixture, high engine temperature, and producing far more emissions.
An oxygen sensor will perform best at approximately 350Â°C. the operating temperature of the sensor is affected by the location in the exhaust: too close to the exhaust manifold can lead to overheating, this will result in burning the heating element or the filaments inside thus emitting harmful gases (CO, HC) in the atmosphere, too far from the exhaust manifold and the sensor may not reach its correct operating temperature, this will result in emitting harmful gases (CO, HC) in the atmosphere.
The location of the ECU also matters. Keeping the ECU in the engine bay can overheat the processor inside the ECU causing delays in input and output signals. The best location for the ECU is under the Passenger compartment, this will keep the temperature low, and it is beneficial for the micro processor to perform its task well.
Earlier example of ECU were not as sophisticated as modern ECU. Older ECUs were used on Single Point Injector system. They were simpler than the modern day ECU, as it used fewer components, and had less function to carry out.
Single Point injection ECU had to only carry out fuel injection; it still used the conventional ignition system, and had very few sensors, and actuators. Against the modern day ECU it has few advantages and disadvantages.
Fewer components therefore cutting cost
Less control over the engine
Less components to go wrong
Fuel is well atomized
Inappropriate fuel distribution
Having less components means it had less control over correct running of the engine, and meeting the emission standards. This major disadvantage lead to the development of a much sophisticated ECU which could be found in modern day vehicle e.g. Multi-Point Injection System.
Multi-Point injection system houses more sensors and actuators than the Single Point injection system. The components used in addition to Single Point injection are mentioned below.
Pre and Post Lambda sensor.
Variable Valve Timing actuator.
Hall Effect Sensor
One major addition to the whole operation of the multi-point injection system is the ignition system. Multi Point injection system uses individual coil per each cylinder, whereas Single Point injection system uses the conventional ignition system. The basic principle of operation for each coil remains the same as the conventional.
By having single coil per cylinder, the time available to build up coil energy is greater than having a single coil providing spark to all cylinders. For example on an eight cylinder engine with eight individual ignition coils, each coil will have 8 x longer time to complete one whole ignition cycle compared to the conventional engine using single coil for all cylinders. The available ââ‚¬Ëœdwellââ‚¬â„¢ time is therefore also up to eight times longer. And there will no wasted spark. Therefore, the car will need no tuning for speed advance, or load advance, and spark will me more efficient.
Multi-Point injection system has more components, therefore having more control over the engine, and meeting the emission standard. It helps the engine to run more cleaner, and perform better, with respect to any weather conditions. It also improves the fuel economy, as distribution of fuel is appropriate to each cylinder.
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